Fiber gratings are incorporated into components that form the backbone of modern information and communications technologies, and are suitable for a wide range of applications, such as information processing and optical fiber communication systems utilizing wavelength division multiplexing (WDM). There are many different fiber grating types and configurations. For example, fiber Bragg gratings are actually one-dimensional photonic stop band structures that are useful in lasing, filtering and sensing applications. Various Bragg grating configurations also include chirped fiber gratings useful in chromatic dispersion compensators and apodized fiber gratings that are used to eliminate sidelobes in signal transmission spectra.
Another type of grating—a long period fiber grating—is of particular interest in sensing and filtering applications, and in other applications in which modulation is achieved by coupling of light into radiating or co-propagating cladding modes rather than by reflection. Unlike a fiber Bragg grating, a long period fiber grating is typically used for coupling the mode of the fiber core into the fiber cladding, which results in attenuation of the transmitted fiber mode. There are multiple transmission dips in the spectrum of a long period fiber grating. These transmission dips occur without reflection of the light signal passing therethrough. The positions of these dips along the spectral range depend strongly on the refractive index of a medium outside the cladding of the fiber. Thus, changing the refractive index outside the fiber produces a shift in the transmission dips. Typically, the period of a long period fiber grating is significantly longer than the wavelength of light passing through the fiber.
The conventional method of manufacturing fiber gratings (including long period fiber gratings) is based on photo-induced changes of the refractive index. Extended lengths of periodic fiber are produced by moving the fiber and re-exposing it to the illumination while carefully aligning the position so that it is in phase with the previously written periodic modulation. The fiber core utilized in the process must be composed of specially prepared photorefractive glass, such as germanium doped silicate glass. This approach limits the length of the resulting grating and also limits the index contrast produced. Furthermore, such equipment requires perfect alignment of the lasers and exact coordination of the fiber over minute distances when it is displaced prior to being exposed again to the laser beam.
It would thus be desirable to provide an advantageous long period fiber grating that has superior properties to previously known long period fiber gratings and that is easy to fabricate.